There are two methods of converting an AC voltage to a DC voltage. A first method uses a diode bridge circuit and a smoothing capacitor. A diode bridge circuit performs full-wave rectification of an alternating current supplied from an AC power supply. A smoothing capacitor smoothes a direct current after full-wave rectification.
In the first method, a current always flows through a series circuit comprising two diodes in either case where an AC voltage is positive or negative. At this time, a power loss equivalent to the product of a current flowing through each diode and a forward voltage of a diode occurs in two diodes.
A second method uses a power factor converter (PFC) between a diode bridge circuit and a smoothing capacitor in the first method. A power factor converter increases the voltage of a direct current after full-wave rectification by a diode bridge circuit.
In the second method, a current flows in a series circuit comprising two diodes during full-wave rectification, and a power loss occurs. In addition, a current flows alternately in a field-effect transistor (FET) and diode comprising a power rector converter, causing additional loss.
Further, in a power factor converter, an output voltage must be set higher than an input voltage to obtain a sinusoidal waveform of an input current. However, a voltage required of a load is not necessarily higher than an input voltage. In such a case, a step-down converter is connected to a later stage of a power factor converter, thereby decreasing the voltage increased by a power factor converter to a desired value. A loss occurs while decreasing the voltage. As a whole unit, a power conversion apparatus comprises three stages of an AC-DC converter, a DC-DC (step-up) converter, and a DC-DC (step-down) converter. A power loss appears as a product of three stages. For example, assuming the efficiency of one stage to be 0.95, 0.95×0.95×0.95=0.86 for three stages. In other words, even the conversion efficiency is as high as 95%, the efficiency decreases to 86% in three stages. As seen above, even if the conversion efficiency of each stage is high, the conversion efficiency extremely decreases in multiple stages.
Nowadays, energy-saving electronic apparatus has been demanded. As a measure of saving energy, it is required to improve the conversion efficiency of a power conversion apparatus for supplying power to a load. However, the improvement of conversion efficiency is limited in a conventional circuit configuration.